When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a "lead") having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as "capturing" the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is the treatment of congestive heart failure (also referred to as "CHF"). Congestive heart failure, which can result from long-term hypertension, is a condition in which the muscle in the walls of at least one of the right and left sides of the heart deteriorates. By way of example, suppose the muscle in the walls of left side of the heart deteriorates. As a result, the left atrium and left ventricle become enlarged, and the heart muscle displays less contractility. This decreases cardiac output of blood through the circulatory system which, in turn, may result in an increased heart rate and less resting time between heartbeats. The heart consumes more energy and oxygen, and its condition typically worsens over a period of time.
In the above example, as the left side of the heart becomes enlarged, the intrinsic electrical heart signals that control heart rhythm are also affected. Normally, such intrinsic signals originate in the sinoatrial (SA) node in the upper right atrium, traveling through and depolarizing the atrial heart tissue such that resulting contractions of the right and left atria are triggered. The intrinsic atrial heart signals are received by the atrioventricular (AV) node which, in turn, triggers a subsequent ventricular intrinsic heart signal that travels through and depolarizes the ventricular heart tissue such that resulting contractions of the right and left ventricles are triggered substantially simultaneously.
In the above example, where the left side of the heart has become enlarged due to congestive heart failure, however, the ventricular intrinsic heart signals may travel through and depolarize the left side of the heart more slowly than in the right side of the heart. As a result, the left and right ventricles do not contract simultaneously, but rather, the left ventricle contracts after the right ventricle. This reduces the pumping efficiency of the heart. Moreover, in the case of left bundle branch block (LBBB), for example, different regions within the left ventricle may not contract together in a coordinated fashion.
Congestive heart failure can be treated by biventricular coordination therapy that provides pacing pulses to both right and left ventricles. See, e.g., Mower U.S. Pat. No. 4,928,688. Congestive heart failure may also result in an overly long atrioventricular (AV) delay between atrial and ventricular contractions, again reducing the pumping efficiency of the heart. There is a need to provide congestive heart failure patients with improved pacing and coordination therapies for improving the AV delay, coordinating ventricular contractions, or otherwise increasing heart pumping efficiency.
Another problem faced by cardiac rhythm management systems is the presence of atrial tachyarrhythmias, such as atrial fibrillation, occurring in patients having congestive heart failure. Atrial fibrillation is a common cardiac arrhythmia that reduces the pumping efficiency of the heart, though not to as great a degree as in ventricular fibrillation. However, this reduced pumping efficiency requires the ventricle to work harder, which is particularly undesirable in congestive heart failure or other sick patients that cannot tolerate additional stresses. Even though a congestive heart failure may have adequate ventricular coordination and cardiac output in the presence of a normal sinus rhythm, when atrial tachyarrhythmia is present, ventricular incoordination may occur, seriously worsening cardiac function.
Moreover, some devices treating congestive heart failure sense atrial heart rate and provide biventricular coordination therapy at a ventricular heart rate that tracks the atrial heart rate. See, e.g., Mower U.S. Pat. No. 4,928,688. Such atrial-tracking devices require a normal sinus rhythm to ensure proper delivery of biventricular coordination therapy. In the presence of atrial tachyarrhythmias, such as atrial fibrillation, however, such atrial tracking biventricular coordination therapy could lead to too-fast and irregular biventricular coordination that is ineffective and even dangerous.
Another problem is that atrial fibrillation may induce irregular ventricular heart rhythms by processes that are yet to be fully understood. Such induced ventricular arrhythmias compromise pumping efficiency even more drastically than atrial arrhythmias. Some devices treating congestive heart failure provide biventricular coordination therapy that does not track the atrial heart rate, but instead, a sensed ventricular contraction in a first ventricle triggers a ventricular pace in the other ventricle, or in both ventricles. See, e.g. Mower U.S. Pat. No. 4,928,688. Even if such biventricular coordination therapy is ventricular-triggered rather than atrial-tracking, the presence of atrial tachyarrhythmias could lead to ventricular arrhythmias, such that the biventricular coordination therapy becomes ineffective and even dangerous because it is too-fast or irregular because of the irregular ventricular heart rate. For these and other reasons, there is a need to provide congestive heart failure patients with improved pacing and coordination therapies for improving the AV delay, coordinating ventricular contractions, or otherwise increasing heart pumping efficiency, even during atrial arrhythmias such as atrial fibrillation.